AQA A Level Chemistry

Revision Notes

8.1.3 Required Practical 9

Required Practical 9

Required Practical 9: Investigating pH changes

The experimental set up for measuring pH changes in titrations

Steps in the procedure

  • Measure out 50 cm3 of 1.0 mol dm-3 ethanoic acid using a measuring cylinder and transfer it to a 200 cm3 beaker
  • Place the beaker onto a magnetic stirrer and add a magnetic stirrer bar
  • Position the pH probe so that it does not interfere with the movement of the stirrer bar
  • Take an initial pH reading
  • Fill the burette with 1.0 mol dm-3 sodium hydroxide
  • Add 2 cm3 of sodium hydroxide from the burette into the beaker and take the pH reading
  • Continue adding 2 cm3 portions until you get to 20 cm3 then switch to 1 cm3 portions
  • The pH begins to rise rapidly- when it begins to slow down then you can switch back to 2 cm3 portions
  • Plot a graph of pH versus volume of base added
  • The procedure can then be repeated with 1.0 mol dm-3 hydrochloric acid and 1.0 mol dm-3 ammonia solution

Practical tips

  • You may need to wait a few seconds until the pH stabilises before you take a reading
  • Make sure the stirrer is not moving too fast as a rapid spin generates bubbles in the solution which gives an unsteady pH reading

Specimen Results

  • The pH curves for a weak acid with a strong base, and a strong acid with a weak base are shown below:

Required Practical 9 Results, downloadable AS & A Level Chemistry revision notes

pH curves from weak acid + strong base, and strong acid + weak base

Analysis

  • The pH curves show a characteristic s-shape curve and the midpoint of the inflection is called the equivalence or stochiometric point
  • From the curves you can
    • Determine the pH of the acid by looking where the curve starts on the y-axis
    • Find the pH at the equivalence point
    • Find the volume of base at the equivalence point
    • Obtain the range of pH at the vertical section of the curve
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